1. Field of the Invention
The present invention relates to image forming apparatuses, for instance, a copier and a printer, that have a function of forming an image on a recording material, such as a sheet.
2. Description of the Related Art
Conventionally, such a type of an image forming apparatus adjusts the rotation velocity of a polygon mirror (starts up a scanner motor) such that the rotation velocity of the polygon mirror driven by the scanner motor is a velocity for performing image forming processing, before print processing onto a recording material. In this case, there is a configuration using the period of a horizontal synchronization signal detected during forcible emission with a laser beam, for detecting the rotation velocity of the polygon mirror.
In this configuration, on startup of the polygon mirror, a laser beam is emitted continuously after startup of the scanner motor, and the horizontal synchronization signal is monitored. Instead, after a time until the scanner motor reaches a target velocity, forcible emission with a laser beam is performed. During forcible emission, the horizontal synchronization signal is detected. According to the period of the detected signal, the rotation velocity of the scanner motor (polygon mirror) is detected, and the velocity is adjusted so as to be a velocity for performing image forming processing. Thus, during forcible emission with the laser beam for detecting the rotation velocity of the polygon mirror, a photosensitive member (image bearing member) is exposed to the laser beam. Accordingly, if a developing roller is in contact with the photosensitive member or a developing bias is applied to the developer, toner adheres to the photosensitive member. Unnecessary toner is transferred to a transfer unit that transfers toner to a recording material directly in contact with the photosensitive member, or a transfer unit that transfers toner to the recording material from the photosensitive member via a belt. There is a fear that, when a recording material is inserted, toner other than for a toner image formed on the recording material adheres to make the material dirty.
Japanese Patent Application Laid-Open No. H07-306626 proposes control that causes a developer to apply a low power bias such that, even if a photosensitive member is exposed to a laser beam, developing is difficult to be performed.
Japanese Patent Application Laid-Open No. 2007-93771 proposes a configuration in an in-line color image forming apparatus including a configuration causing a developer to contact with and separate from a photosensitive member; the apparatus stands by in a state where the developer is separated from a photosensitive member to prevent toner from adhering the photosensitive member, and brings the developer into contact immediately before image forming.
U.S. Pat. No. 5,864,355 proposes that a safety area is provided such that a photosensitive member is not exposed to a laser beam and the area does not overlap with an image area on a surface of the photosensitive member, and, on startup of a scanner motor, only the safety area is emitted with the laser beam and calibration is performed.
However, in the case where a low power bias is applied to the developer as with Japanese Patent Application Laid-Open No. H07-306626 to prevent the undersurface of a recording material from being made dirty by emission on the photosensitive member with a laser beam, a time is required for switching from the low power bias to a normal bias. The image forming processing cannot be operated immediate after completion of forcible emission with the laser beam. Accordingly, there is a fear of extending a first print out time (FPOT), which is a time from reception of a printing request to ejection of a first sheet of a recording material out of an apparatus. There is another fear that, in the case of exposure to a laser beam having a significantly high light intensity, a certain amount of toner adheres to a photosensitive member and developed even with a low power bias application to make the underside of a recording material dirty.
As with Japanese Patent Application Laid-Open No. 2007-93771, in the case where the developer waits in a state of separating from the photosensitive member to prevent toner from adhering the photosensitive member and the developer is brought into contact with the photosensitive member immediately before image forming, timing when the developer is brought into contact with the photosensitive member should be after the scanner motor starts up and reaches a prescribed rotation velocity for image forming processing and the state transitions to that without unexpected exposure of the photosensitive member. Accordingly, there is a fear of extending the first print out time as with the above description.
Furthermore, in the case as with U.S. Pat. No. 5,864,355 where an area that does not overlap with an image area on the surface of the photosensitive member is calculated and acquired and a laser beam is emitted, it is difficult to follow abrupt variation in velocity of the polygon mirror. There is a fear of exposing the image area on the surface of the photosensitive member. In the case where a horizontal synchronization signal cannot be acquired in the area that does not overlap with the image area on the surface of the photosensitive member, there is a fear that the scanner motor cannot be controlled. In order not to abruptly vary the speed of the polygon mirror, the acceleration of the polygon mirror is required to be kept within a certain range. In this case, a time is required until adjustment of the velocity of the polygon mirror is completed. There is a fear of extending the first print out time.